Aromatic alkyls are commonly catalytically oxidized to aromatic carboxylic acids, in the liquid phase, within a pressurized oxidation reactor. Such liquid-phase reaction systems are shown in U.S. Pat. Nos. 3,170,768 and 3,092,658, both to Baldwin. The reaction medium contained within the reactor typically includes the aromatic alkyl, a volatilizable aqueous solvent, an oxygen-containing gas, and the oxidation catalyst.
The oxidation reaction is exothermic. A substantial portion of the reaction-generated heat is removed by evaporating a portion of the reaction mixture from the reactor, withdrawing this portion from the reactor as a reactor overhead vapor stream, partially condensing this vapor stream and returning the condensate to the reactor. The remainder of the vapor stream is conventionally passed through an absorber system to recover unreacted aromatic alkyl and solvent.
A product stream, containing the aromatic carboxylic acid product solvent, oxidation reaction by-products and catalyst, is withdrawn from the reactor and is passed through crystallizers. A typical product stream contains about 1-6 parts by weight of a liquid phase containing about 15 weight percent of water and about 85 weight percent solvent, typically acetic acid, and about one part by weight of the aromatic carboxylic acid product. The crystallizers concentrate the aqueous product stream by removing a portion of the water, solvent, and volatilizable by-products. The thus-concentrated product stream, exiting the crystallizers and bearing crystals of product, contains the remainder of the solvent, the remainder of the reaction by-products, and the catalyst. These by-products together with the catalyst are commonly referred to as oxidation residue. The concentrated product stream is separated into aromatic carboxylic acid product crystals and an aqueous aromatic carboxylic acid product mother liquor stream.
Conventionally, at least a portion of the product mother liquor stream is passed through a solvent recovery system to separate the residue from the solvent. The residue is typically combined with an extraneous water stream to produce an extraction slurry. The extraction slurry is then separated into so-called waste solids and a catalyst-containing mother liquor, which is thereafter concentrated in an evaporation step. Finally, the concentrated catalyst-containing mother liquor is recycled to the reactor.
The conventional catalyst-concentrating step is typically achieved via an evaporative system that entails substantial capital investment. The variable costs of the evaporation system, moreover, include steam, cooling water, and electrical costs. The evaporaation system equipment includes evaporator reboilers, condensers, separators, and a variety of pumps and agitated vessels. There are also maintenance expenses associated with this equipment.
The present invention is a catalyst-recovery method which results in significantly lower capital and variable costs in the overall oxidation process as compared to conventional processes. In particular, the present catalyst-recovery method renders unnecessary substantially all of the above-enumerated catalyst-recovery evaporation system equipment, and its associated maintenance and operating expenses that are typically required in connection with conventional processes.